Best way to model this mushroom
I would probably just sculpt that detail in sculpt mode with dyntopo enabled and detail size set to 3 - 5. I'd probably make the hat of the mushroom a separate object in order not to mess up the nice topology of the rest of the model. Also, if you keep a copy of the hat as it is now, you could then later use the high-res sculpted hat to bake displacement and/or normal maps to the low-res hat, if you're concerned about performance or render times for your final scene.